1. Field of the Invention
The present invention relates to an air fuel ratio control apparatus for an internal combustion engine.
2. Description of the Related Art
There has hitherto been known a technique in which an NOx storage reduction catalyst (hereinafter referred to as “NOx catalyst”) is disposed on an exhaust passage of an internal combustion engine, so that nitrogen oxides (NOx) in an exhaust gas is occluded into the NOx catalyst when the exhaust gas is in an oxidative atmosphere, whereas the NOx occluded in the NOx catalyst is reduced to purify the NOx in the exhaust gas when the exhaust gas becomes a reducing atmosphere.
In addition, another technique has also been known in which similar to the case of NOx, poisoning of an NOx catalyst due to sulfur components occluded therein is recovered by decreasing the air fuel ratio of a mixture after the temperature of the NOx catalyst has been raised (see, for example, Patent Document 1: Japanese patent application laid-open No. 11-44211).
Here, note that there are following documents that are relevant to the present invention.
Patent Document 2: Japanese patent application laid-open No. 2002-89350
Patent document 3: Japanese patent application laid-open No. 7-189797
Patent document 4: Japanese patent application laid-open No. 2002-188430
Incidentally, sulfur poisoning recovery processing can be performed most efficiently and quickly when the temperature of the NOx catalyst and the condition of the air fuel ratio in the NOx catalyst are optimal.
In this regard, it becomes possible to obtain an optimal temperature of the NOx catalyst and an optimal air fuel ratio of the mixture by detecting the temperature of the NOx catalyst and the air fuel ratio of the mixture by means of sensors thereby to control the amount of fuel to be added in a feedback manner, and by further learning the feedback value of the added fuel amount thus feedback controlled, but there might be cases where the temperatures of the NOx catalyst and the air fuel mixture are not able to be detected in an accurate manner due to the characteristics of the sensors.